The concept of Random Access (RA) is used in cellular networks e.g. in order for mobile terminals (UEs), which are not yet known to the network, to establish contact with a cellular network. The possibility for a UE to request a connection setup by RA is a fundamental requirement for any cellular system. For example, in LTE, RA is used for several purposes, including:                for initial access when establishing a radio link;        to re-establish a radio link after radio-link failure;        for handover when uplink synchronization needs to be established to the new cell;        to establish uplink synchronization if uplink or downlink data arrives when the UE is in RRC_CONNECTED state, and the uplink is not synchronized;        for the purpose of positioning using positioning methods based on uplink measurements; and        as a scheduling request if no dedicated scheduling request resources have been configured on PUCCH (Physical Uplink Control Channel).LTE RA is described in more detail e.g. in 3GPP TS 36.213, chapter 6, and in 3GPP TS 36.321, section 5.1.        
It is vital for cellular system performance and accessibility that the RA procedure results in a successful contact with the network, and preferably without any significant delays, which may cause user dissatisfaction.
In a cellular network, there may be areas with “high traffic”, i.e. a high concentration of users. An exemplifying cell 100 comprising areas 103 with a high concentration of users is illustrated in FIG. 1a. In such high traffic areas 103 it may be desired to deploy additional capacity in order e.g. to keep the user satisfaction. Capacity could be added in the form of an additional macro base station, generating/serving a cell which covers one or more of the area(s) in need of extra capacity. Capacity could also be added in the form of additional nodes with lower output power, as compared to a macro base station, and thus covering a relatively smaller area, to which the desired capacity boost is concentrated.
There may also be areas, e.g. within a macro cell, with unfavorable radio conditions or “bad coverage”, where there may be a need for coverage extension. One way to achieve a coverage extension is to deploy an additional node, e.g. a node with a low output power, which concentrates the coverage boost to a relatively small area, e.g. where it is most needed.
One argument for choosing nodes with lower output power for increasing capacity or coverage as in the above cases is that the impact on the “original” macro nodes/network can be minimized. That is, by that the interference to an “original” macro node, with a coverage which at least partially overlaps the coverage of the “added” lower output power node, may be limited to a relatively small area.
FIG. 1b illustrates a macro base station 102, which provides a wide area coverage 100 (also called macro cell). FIG. 1b also shows examples of low power nodes that are deployed to provide small area capacity/coverage. In this example, pico base stations 104, relays 108 and home base stations 110 (femto cells) are shown. A pico base station can either be similar to a macro eNB, but typically with more limited coverage, for example, having a lower max transmission power, or, be a remote radio unit connected to a main unit. A common term for such pico/relay/femto cells is “underlay cells”, served by “underlay nodes”. This type of network deployments are typically referred to as: “Heterogeneous Networks”, “multilayer networks” or shortly “HetNets”. FIG. 1b shows clusters of femto cells; however, single femto cell deployments are also possible. The cells can be either so-called “open access”, or provide access only to a Closed Subscription Group (CSG).
Underlay cells typically operate at lower reference (pilot/perch) signal powers, as compared to macro cells. This means that if the cell selections as well as mobility decisions are based on received reference signal strengths, the downlink cell border will be located closer to the underlay node than to the macro node/eNB. If the uplink sensitivity for all cells is similar, or if the difference in uplink sensitivity is not equivalent to the difference in reference (pilot/perch) signal powers, then the uplink cell border will be different from the downlink cell border.
FIG. 2 illustrates a scenario where the uplink and downlink cell borders are separated. A situation where the UL and DL borders are separated may be referred to as an uplink/downlink (or downlink/uplink) imbalance in the area between the separated borders. The situation of uplink/downlink imbalance is not limited to macro cell/underlay cell combinations, but may arise also between macro cells and in locations with unfavorable radio conditions, e.g. in urban environments.
Referring to the example illustrated in FIG. 2, a first UE served by the macro node may cause significant uplink interference to the underlay node if located in an area relatively close to the underlay node. In fact, if located in the area with uplink/downlink imbalance, said UE may even have the best uplink to the underlay node/cell, but might nonetheless not have detected the underlay cell reference signal.
If a second UE, which is not yet known to the network, attempts to perform a RA to the underlay node/cell in the presence of UEs such as the first UE, the underlay node may have difficulties distinguishing the RA request from the uplink interference generated by the UEs served by the macro node. Consequently, the RA attempts of the second UE may fail repeatedly.
One way to relieve this situation of significant interference to the underlay node is to consider an underlay cell range expansions by considering offsets in the cell selection and/or mobility decisions. Thereby, potentially interfering UEs served by the macro node will be at a longer distance away from the underlay node, and thereby induce less interference to the underlay node. However, this also means that some UEs served by the underlay node can be subjected to critical interference from the macro node in the downlink.